The present invention relates to the general field of protective coatings for parts used in high-temperature environments. The invention applies more particularly, but not exclusively, to the thermal barriers used to protect superalloy parts in aviation gas turbines.
The parts present in the hot portions of aviation turbine engines operating in desert environments or in very polluted environments can become damaged because of attack by sand or by alkaline compounds present in the air ingested by the engine. These compounds, which include in particular calcium and magnesium aluminosilicates (CMAS), can degrade the thermal barrier layer covering certain parts in the hot portions of the turbine engine.
The mechanisms whereby CMAS particles degrade the thermal barrier include in particular CMAS infiltrating in the liquid state into the thermal barrier layer, and the thermal barrier layer (which is conventionally constituted by a ceramic based on yttria-stabilized zirconia (YSZ)) being dissolved and re-precipitating in the form of yttria-poor zirconia. Those two mechanisms reduce the mechanical properties of the thermal barrier layer and can lead to it cracking during stages of the engine cooling. Furthermore, ingesting solid particles gives rise to erosion phenomena of the thermal barrier layer which spalls and lays bare the underlying substrate, thus reducing the lifetime of the parts.
Solutions exist for limiting the infiltration of CMAS into the thermal barrier layer. By way of example, mention may be made of using a coating for protecting the thermal barrier that is based on gadolinium-doped zirconia (e.g. referred to as gadolinium zirconate), or indeed using alumina or titanium oxide. On reacting with CMAS, those coatings encourage precipitation of CMAS and thus serve to limit penetration thereof into the thermal barrier. Nevertheless, those coatings present the drawback of being sacrificial, which requires permanent maintenance, and also regular monitoring of the state of the parts.
There therefore exists a need to have a coating for protecting parts from CMAS that presents a lifetime that is long, in particular in the operating environments and conditions of an aviation turbine engine.